A bit of storytime, if I may..
Much like you, I started as a vassal to the elves. Queen Arwen was already at war with Hakkon Empire, and I had enough renown and persuasion to get Serin Castle for myself. Then, I amassed a nice huge army of Household Guards and Hired Knights, and led them to conquer Aruolo... We weren't at war with them, but I didn't let that stop me, dishonorably conquered Qualetaga (which was ridiculously easy) and gained about 900 prisoners in that siege alone.. then I claimed Qualetaga for myself, and prepared my campaign against the Geldarin.
By the time Zann invaded, I was the lord of Qaletaga, Azure Keep, and Ianthas Castle. The latter I got by exchanging Serin Castle for it. So I had carved a nice little corner of the kingdom all by myself, and by day 150 I had a respectable amount of Household Guards, Hired Knights, Elintor Knights and Geldarin Dukes.. the latter two provided me with boosts to pathfinding and morale.
I allowed the Zann to take Forniron, and convinced Queen Arwen to make me marshal. I took it in a bloody siege, sacrificing many elven allies in process.. But when we were victorious, I demanded if for myself, and persuaded most of the lords to support me.. Thus, Queen Arwen was left fiefless, and I now owned the majority of the land in Elintor.
With Elintor getting the worst from the Zann, I renounced my oath, and left Arwen stranded between me and the invaders. I constantly harassed their lords in the field, captured the Queen herself, and let Zann take huge bites out of Elintor pie until I felt confident enough to take Elsinore. The battle for Elsinore was even bloodier than for Forniron, and we took huge casualties (mainly because of the map itself
)
With Elsinore in my hands, Princess Lithrandil was easily convinced to join me, and soon Arwen had only Lille left.. at which point I released her from my prison and gave her a truce of 20 days, just to keep things interesting.
Here's what it looks like now (day 292): pink is Perisno, green is Zann.